Tag Archives: Arkansas

Sometimes the best way to see what’s on the ground is to get as far away from it as possible. Geologists use a variety of tools to do just that. One of those tools is aerial photography.

The picture above shows the Arkansas River where it leaves the mountainous western uplands and enters the bottomlands of the Mississippi Alluvial Plain, east of Little Rock (upper left). Driving east across that boundary, it’s easy to get the impression that the Mississippi Alluvial Plain is a broad flat expanse of land with little to no distinguishing features. That’s far from the case as this photo reveals.

From high-altitude imagery, subtle relic features created by the Arkansas River can be easily recognized. Note the swirling landforms that characterize the lowlands on the right side of the picture. Over time the Arkansas River has meandered through the valley carving new channel courses and abandoning old reaches of channel. The continually changing river has left a mosaic of oxbow lakes (water-filled abandoned channels) and arc-shaped river deposits known as point bars.

The mineral in the above pictures is calcite, a common mineral in earth’s crust that is the main component of the sedimentary rock limestone. The stack of samples (top) exhibit a physical characteristic known as cleavage. The cleavage of calcite causes it to break into a rhombus-shape (see picture).

Cleavage is the tendency of a crystalline substance, such as a mineral, to break along parallel planes that reflect the internal arrangement of the atoms in the crystal. All crystals, by definition, have a uniform atomic arrangement. To illustrate this property, I’ve included a second picture (bottom), borrowed from Dr. Cathy Sutton, that shows an extremely magnified calcite crystal. The repeating rhombus-shapes in the picture are individual calcite molecules. Basically, cleavage is the outward expression of the internal structure of a mineral.

The samples on the left were collected from Midwest Lime Quarry, Batesville, Arkansas.

The photo above shows trace fossils that record the travels of two trilobites. Trilobites are an extinct group of marine invertebrate animals, resembling horse-shoe crabs, that flourished for 100s of millions of years in the Paleozoic Era (540-250 mya). The tracks the animal left are known as the trace fossil, Cruziana. It appears that one traveled from the right side of the photo, the other from the left, until they met in the middle where they rested for a while. At the center of the photo are resting traces known as Rusophycus. Perhaps they became friends or maybe they were even more than friends? It is Valentine’s Day. Their traces are preserved in the Atoka Formation of west-central Arkansas.

We are celebrating the 25th year of detailed geologic mapping in Arkansas made possible by the passage of the National Geologic Mapping Act of 1992. It established STATEMAP which distributes funds to the states, typically geological surveys, in the form of cooperative grants which are used to partially fund various geologic mapping projects. The first grant received by the Arkansas Geological Survey, then known as the Arkansas Geological Commission, was for a proposal in fiscal year 1994. Since that time, seventy-eight 1:24,000-scale geologic maps have been completed, with two more on the way this year. Two maps at the 1:100,000-scale have also been published. This marks an unprecedented commitment to gathering data about the surface of the earth in our state. Following is a factsheet summarizing the STATEMAP projects in Arkansas since 1994.

Here is the law establishing STATEMAP:

National Geologic Mapping Act of 1992

PUBLIC LAW 102-285

102d Congress

signed May 18, 1992

An Act

To enhance geologic mapping of the United States, and for other purposes.

Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled,

43 USC section 31a. Findings and purpose

(a) Findings

The Congress finds and declares that–

(1) during the past 2 decades, the production of geologic maps has been drastically curtailed;

(F) design and construction of infrastructure requirements such as utility lifelines, transportation corridors, and surface-water impoundments;

(G) reducing losses from landslides and other ground failures;

(H) mitigating effects of coastal and stream erosion;

(I) siting of critical facilities; and

(J) basic earth-science research;

(3) Federal agencies, State and local governments, private industry, and the general public depend on the information provided by geologic maps to determine the extent of potential environmental damage before embarking on projects that could lead to preventable, costly environmental problems or litigation;

(4) the combined capabilities of State, Federal, and academic groups to provide geologic mapping are not sufficient to meet the present and future needs of the United States for national security, environmental protection, and energy self-sufficiency of the Nation;

(5) States are willing to contribute 50 percent of the funding necessary to complete the mapping of the geology within the State;

(6) the lack of proper geologic maps has led to the poor design of such structures as dams and waste-disposal facilities;

(7) geologic maps have proven indispensable in the search for needed fossil-fuel and mineral resources; and

(8) a comprehensive nationwide program of geologic mapping is required in order to systematically build the Nation’s geologic-map data base at a pace that responds to increasing demand.

(b) Purpose

The purpose of sections 31a to 31h of this title is to expedite the production of a geologic-map data base for the Nation, to be located within the United States Geological Survey, which can be applied to land-use management, assessment, and utilization, conservation of natural resources, groundwater management, and environmental protection.

section 31c. Geologic mapping program

(c) Program objectives

The objectives of the geologic mapping program shall include–

(1) determining the Nation’s geologic framework through systematic development of geologic maps at scales appropriate to the geologic setting and the perceived applications, such maps to be contributed to the national geologic map data base;

(2) development of a complementary national geophysical-map data base, geochemical-map data base, and a geochronologic and paleontologic data base that provide value-added descriptive and interpretive information to the geologic-map data base;

(3) application of cost-effective mapping techniques that assemble, produce, translate and disseminate geologic-map information and that render such information of greater application and benefit to the public; and

(4) development of public awareness for the role and application of geologic-map information to the resolution of national issues of land use management.

(d) Program components

(3) A State geologic mapping component, whose objective shall be determining the geologic framework of areas that the State geological surveys determine to be vital to the economic, social, or scientific welfare of individual States. Mapping priorities shall be determined by multirepresentational State panels and shall be integrated with national priorities. Federal funding for the State component shall be matched on a one-to-one basis with non-Federal funds.

This rather handsome outcrop of the Wilcox group consists of alternating layers of sand and clay of the Eocene Epoch which lasted from about 56-34 million years ago. The Wilcox Group is a non-marine unit mostly composed of sand with lesser clay, silt, gravel, and lignite (low-grade coal).

This geologic unit is part of a larger sequence of loosely-consolidated sedimentary rocks exposed in south central Arkansas, south of Pulaski county. These rocks are the northern extent of the West Gulf Coastal Plain, a physiographic province that stretches from central Arkansas, south, to the Gulf of Mexico.

In the picture above, large black rectangular aegerine crystals are prominent in a rock type known as a pegmatite. Pegmatites are igneous rocks characterized by extremely large crystals. Sometimes they also contain unusual mineral species. This sample was collected from Magnet Cove, Arkansas. Magnet Cove, which is approximately 10 miles east of Hot Springs, is one of the few places in Arkansas where igneous rock is exposed at the surface.

Between 84 and 100 million years ago, magma was injected into the earth’s crust under central Arkansas where it slowly cooled and crystallized into igneous rock. Millions of years of erosion eventually unearthed that rock. Despite only being exposed over approximately 5 square miles, the rocks of Magnet Cove have yielded more than 100 different minerals. Rare minerals have been discovered there including a new variety of zirconium-rich garnet called Kimzeyite.

Why do rocks have beds? Are rock beds where geologists sleep? Sometimes, but that’s not the point of this article. The picture above, taken on the Goat Trail at Big Bluff, overlooking the Buffalo National River, is a great example of a sedimentary rock composed of many individual beds (layers). The reason that rocks are bedded is due to either gaps in deposition or abrupt changes in the grain size of sediment being deposited in an environment.

Here’s an example; when a storm causes a river to flood its valley, the water deposits sediment as the flood recedes. Typically, there’s a period of non-deposition before the next flood event deposits a new layer of sediment over that one. This time between floods allows weathering to alter the character of the first flood deposit. That weathered surface will eventually differentiate the flood deposits into distinct beds of rock.

Bedding can also form as a result of flowing water gaining or losing velocity. The size of sediment that water carries (and eventually deposits) is directly related to flow rate. A sudden change in flow rate creates bedding distinguished by differences in grain size.

Everyone in the photo above was eventually air-lifted to safety… Just kidding! They’re still up there